1. Field of the Invention
The present invention relates to linear position transducers of the magnetostrictive type. More particularly, the exemplary transducer; permits the user to arbitrarily assign set points defining the active measurement stroke of the transducer; permits the user to assign analog output voltages to the set points. The exemplary transducer also incorporates a velocity dependant inhibit window to improve transducer performance in the presence of noise.
2. Description of the Prior Art
Magnetostrictive linear position transducers, are robust, high resolution position indication instruments which have proven to be useful in many measurement and control applications. Commercial magnetostrictive transducers have been available with 0-10 Volt, and 4-20 mA, analog signal outputs. Transducers with timed TTL pulse outputs have also been available. In these devices, the analog signal level or analog time interval indicates the measured distance. It is expected that magnetostrictive devices will become more commonly used in applications which currently rely on other technologies to transduce position into an analog output voltage. Adoption of magnetostrictive transducers will be facilitated by providing sensors which emulate or mimic industry standard voltage output interface protocols, as taught by the present invention. Examples of pertinent prior art include:
U.S. Pat. No. 3,898,555 to Tellerman, which discloses a basic magnetostrictive linear position measurement system. A position indicating magnet is positioned along the length of a tubular magnetostrictive waveguide. A pulse generator is used to generate a current pulse in the waveguide. The position magnet interacts with the magnetic field generated around the waveguide by the current pulse. The interaction generates an acoustic reaction wave in the waveguide at the location of the position magnet. This acoustic or sonic pulse is propagated along the waveguide. The time of flight of the acoustic wave, indicates the position of the magnet with respect to a "mode converter" which is attached to the waveguide and which translates the energy of the acoustic echo pulse into an electrical signal. In Tellerman the time intervals between excitation of the waveguide to the detection of the echo controls the application of a DC reference voltage to an analog filter system which generates an averaged DC level indicating position. In this fashion the location of the magnet is converted into an output DC voltage level which indicates the magnet position.
U.S. Pat. No. 4,721,902 to Tellerman, teaches, inter alia, a method of increasing the resolution of the position indication by a technique now called "recirculation". In operation the echo pulse is used to initiate the next interrogation or excitation pulse which is applied to the waveguide. In this sense the return echo are recirculated. The time measurements of multiple recirculations can be averaged to enhance resolution.
This Tellerman patent also teaches the use of a counter based time interval measurement system. In general, the interrogation pulse starts a counter which collects counts from a clock source. The counting process is stopped with a return echo. In this fashion, the time of flight of the sonic pulse is translated into a representative "raw" count. This "raw" count is then transcribed into a corresponding analog voltage.
This Tellerman patent also teaches a time domain filtering technique which sets the duration of an "inhibit timer" based upon the historical output of the transducer. In general, the raw count data is used to set an "inhibit" time interval which is slightly shorter than the expected echo delay time. The output of the mode converter is ignored until the inhibit time has elapsed. This inhibit time is not velocity dependent although it does vary as a function of magnet position. In practice this time domain filtering technique limits the maximum slew rate of the magnet.
However these representative magnetostrictive transducers do not exhibit the ability to arbitrarily assign the output signal voltage to an arbitrary selected portion of the waveguide. The ability to select the active stroke of the transducer is important in many applications. The ability to assign output voltages improves the adaptability of the transducer and permits it to emulate transducers which it replaces. The velocity adaptive inhibit window allows the transducer to accommodate its noise window to the particular application.